Eric Holder said there would be respect for the rule of law and no political favouritism in the US Justice Department as he was sworn in on Tuesday as the country's first African-American attorney general.
Holder made no specific mention of the former Bush administration, which was criticised by rights groups for a heavy-handed approach to terrorism and for politicised appointments of US prosecutors.
Holder promised a return to the values of "fairness" and a "just America" as he was sworn in as the country's top law enforcement officer.
"This may be a break from the immediate past but it is consistent with the long history of this department," Holder said in a ceremony, after he was administered the oath of office by Vice President Joe Biden.
Holder, who served as deputy attorney general under former president Bill Clinton, was confirmed by the US Senate Monday. He is one of the last major members of President Barack Obama's cabinet to be confirmed.